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encodes homologs to “ionotropic” glutamate receptors, which encode receptor ion channels
in animal systems [115].
Calcium ions act as a second messenger in intracellular signal transduction during ABA
signaling [132]. In-flow of calcium ions into the cytosol from the vacuole and extracellular
space increases the cytosolic concentration of calcium ions in ABA-treated guard cells. The
level of calcium ions oscillates at intervals of several minutes. This increase in calcium
concentration is not observed in the ABA insensitive mutant's abi1 and abi2 [133]. Calcium
ions suppress inward potassium channels and activate inward anion channels; thereby playing
a central role in stomatal closure [134]. Also, the active oxygen species formed activate the
calcium ion channel to increase the cytosolic concentration of calcium ions.
Uptake and distribution of sodium ions within the root is to a large extent connected with
the effects of potassium, since Na
+
efflux in root cortex cells is stimulated by K
+
influx, which
is related to the K/Na root selectivity [135]. The presence of potassium (and calcium) ions
has been shown to decrease Na+ influx into plant cells (e.g. [136]). Potassium promotes cell
elongation and maintains osmoregulation. Potassium promotes photosynthetic rate and
controls the rate of transport of photosynthates from source to sink. Potassium is also
essential for protein synthesis and activates nearly 45 enzymes involved in various metabol‐
ic processes [222].
We observed differential responses in the uptake of sodium and in the pattern of germination
in seedlings of walnut cultivars which could account for the differences in response to salinity.
The ability to maintain low sodium concentration in leaves and in growing shoots is crucial
for plant growth in saline media. The salt tolerance in species that exclude salts is achieved by
changes between sodium and calcium ions, rather than changes in osmotic potential, since
adsorption of calcium ions on membranes of root cells leads to reduced penetration of
monovalent cations [124]. This was demonstrated for wheat where inhibition of non-direc‐
tional Na
+
influx occurred following the addition of external Ca
2+
[137]. Involvement of both
Ca
2+
sensitive and Ca
2+
insensitive pathways (regulated mainly by non-selective cation
channels) in the control of Na
+
entry into the root has been proposed [138]. When sodium
accumulates in the cytoplasm of shoot or leaf cells, it can lead to tissue necrosis and leaf
abscission; thus, the photosynthetic apparatus is impaired and plant growth is hindered. The
accumulation of sodium in shoots was significantly different in the three salt tolerance classes,
but they presented distinct responses to the increasing concentration of NaCl. Similarly, Sixto
et al [116] observed differences in leaf sodium content among
P. alba
cultivars from different
Spanish origins subjected to salt stress. Possibly the halfsib seedlings of 'Chandler', which
accumulated significantly less sodium in shoots, has mechanisms for sodium exclusion at the
root level, which reduces sodium uptake and its translocation to the shoot tissues. Mechanisms
for sodium exclusion in roots are well studied in
P. euphratica
[117] which is the most salt-
tolerant poplar species. In
P. alba
, the ability to maintain lower sodium content in leaves has
also been associated with less severe symptoms of salinity stress [116]. Our results confirm a
negative relationship between sodium accumulation in the shoots and its effects on shoot
growth in 'Chandler'. The negative effect of long-term salt stress on shoot growth of 'Lara' is
probably more due to sodium toxicity than to osmotic effect. The excess sodium can be both
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